Update issue templates

[brooke-hamilton]

Benefit/Result/Outcome
So that the purpose and terminology of the issue templates is more clear.

Description
The "Issue" type of GitHub issue is a confusing name, and causes difficulty in separating what is a general issue, i.e., bug, question, enhancement, etc., from a Issue type of issue, which is planned work.

The "feature request" and "enhancement" issue types are redundant.

Acceptance Criteria

• The "Issue" issue type is changed to "Backlog Item"
• Enhancement is removed.